The invention relates to a brake master cylinder for a motor vehicle.
The invention relates more specifically to a brake master cylinder for a motor vehicle, of the type which comprises a substantially axial body inside a bore of which is slideably mounted at least one axial piston which is capable of being actuated by a driver of the vehicle between a rear rest position and a forward braking force application position, and which is returned elastically toward its rear rest position against an end stop formed in the body, of the type in which the bore comprises two seals, front and rear, which are interposed between the piston and the bore, the front seal delimiting, in the bore, a rear supply chamber and a front pressure chamber, of the type in which the body comprises a radial supply duct which connects an external hydraulic fluid reservoir to the rear supply chamber and which opens out between the two seals, of the type in which the body comprises a braking circuit supply hole which opens into the front pressure chamber, of the type in which the piston comprises a bore, open to the front, communicating on the one hand with the front pressure chamber and on the other hand with the periphery of said piston by way of at least one hole which, when the piston occupies its rear rest position, is arranged between the two seals so as to open communication between the front pressure chamber and the rear supply chamber and which, when the piston is moved axially forward toward its application position, is capable of passing beyond the front seal in order to isolate the front pressure chamber from the rear supply chamber and thus make it possible to establish a braking pressure in the front pressure chamber, of the type in which at least each front seal comprises at least one lip, arranged in contact with the periphery of the piston, which is capable, when the front pressure chamber is exposed to a partial vacuum caused by the return of the piston from its forward application position to its rest position or else by the activation of a trajectory control device forming part of the braking circuit, of separating from the periphery of the piston so as to respectively allow to be resupplied the reservoir by means of the front pressure chamber or else the front pressure chamber to be resupplied by the reservoir.
Many examples of brake master cylinders of this type are known.
In most master cylinders of this type, the piston is generally guided in the bore by way of at least two annular bearing surfaces of the bore, these surfaces being respectively arranged in front of the front seal and to the rear of the rear seal.
The bearing surface which is arranged in front of the front seal adjoins the retaining groove which accommodates the front seal and it comprises so-called resupply grooves which are intended to allow brake fluid to flow through these grooves when the front pressure chamber is exposed to a partial vacuum caused by the return of the piston from its forward application position to its rest position or else when an “ESP”-type trajectory control device of the braking circuit is activated and generates a partial vacuum in the front pressure chamber.
In these conditions, the front seal separates from the periphery of the piston under the effect of the partial vacuum to which it is exposed, thereby respectively allowing the reservoir to be resupplied by means of the front pressure chamber or else the front pressure chamber to be resupplied by the reservoir.
Conventionally, the resupply grooves are produced in the form of a plurality of parallel helical grooves of reduced size, this size being imposed by the reduced length of the bearing surface which is arranged in front of the front seal.
What has been found in many cases, however, is that the brake fluid flow rate is insufficient for resupply to proceed correctly.
This is particularly evident when the vehicle is equipped with an “ESP”-type trajectory control device, in which case the demand for brake fluid, which is intended to supply one or more of the brake calipers of the vehicle, can be high even when the piston is occupying an application position in the pressure chamber. An insufficient flow rate can lead to a lack of efficiency, or at least to a detrimentally slow reaction of the “ESP”-type trajectory control device.
To overcome this disadvantage, the invention provides a brake master cylinder comprising means for increasing the resupply flow rate.
To this end, the invention provides a brake master cylinder of the above-described type, characterized in that it comprises at least one wide groove which is formed in the bore in front of a groove for retaining the front seal, which extends at least in part around the piston, and which communicates with said retaining groove and the hole in the piston, in order to allow the resupplies to occur with a high flow rate.
According to other features of the invention: the groove is a helical groove with a large specified pitch, the helical groove extends axially over a distance which is less than the specified pitch, the groove opens directly into the retaining groove for the front seal, the master cylinder comprises at most two wide grooves associated with the retaining groove for the front seal and with the piston, the master cylinder is of the tandem type and it comprises, in its substantially axial body, a bore inside which two axial pistons are slideably mounted, and two front seals, and in which are formed at least two and at most four associated grooves.